1. Technical Field
The invention relates to brake systems of heavy duty vehicles, and in particular to cam assemblies of the brake system. More particularly, the invention is directed to a cam tube bracket for a cam shaft support/enclosure assembly which enables efficient installation, increases load capacity, and offers increased resistance to torsional and axial deflection of a cam tube of the cam shaft support/enclosure assembly, thus reducing fretting and providing a more secure attachment of the cam tube to a suspension assembly, as well as extending the life of the cam shaft support/enclosure assembly.
2. Background Art
In conventional heavy duty vehicle brake systems, an S-cam is utilized to lift brake shoes against a brake drum to decelerate a vehicle. The S-cam is integrally connected to a cam shaft, which typically is supported at each of its ends by a bushing or bearing. These bushings usually are lubricated or greased to reduce friction between the bushings and the cam shaft.
If the bushings or the cam shaft become worn and/or rotational friction of the cam shaft against the bushings increases, the overall efficiency of the brake system decreases. Of course, when bushing or cam shaft wear exceeds predetermined limits, bushing and/or cam shaft replacement is required. Recommended practice in the industry is that when the cam shaft and/or bushings on one end of an axle require replacement, then the cam shaft and/or bushings on the other end of the same axle should also be replaced, and the cam shafts and/ox bushings on all other axles of the vehicle should be inspected for the same wear condition
There are two primary contributors to cam shaft and bushing wear, namely, load-induced wear and contamination-induced wear Wear due to loading is encountered in two different conditions that can be defined as quasi-static and dynamic. The quasi-static case occurs when the vehicle brakes are applied and braking forces are reacted in the cam shaft bushings. This scenario is considered quasi-static due to the relatively low rotational speed of the cam shaft and the steady state condition when the brakes are held at constant pressure. In such a case, the loads on the outboard bushing are greater than on the inboard bushing The dynamic load case, on the other hand, typically occurs when the brakes are in the released condition and the cam shaft experiences vibrations due to road inputs. These vibrations result in impact loading of the cam shaft against the bushings
Cam shaft and bushing wear due to contamination is caused primarily by environmental factors. To reduce such contamination, seals generally are disposed on each end of each bushing, which capture the lubricant inside the bushings and limit ingress of contaminants from the outside environment. The lubricant not only acts to reduce friction between the cam shaft and the bushings, but also suspends any contaminants that may migrate past the seals into the bushing The lubricant also acts as a barrier to moisture that could cause corrosion of the cam shaft
Thus, various types of cam shaft support/enclosure assemblies have been utilized in the brake system art to protect and support the cam shaft, and to ensure coaxiality of the bushings to prevent excessive rotational friction and/or binding of the cam shaft against one or more of the bushings when the brakes are actuated Maintaining coaxiality of the bushings also improves the load support of the bushings. More particularly, when the bushings remain coaxial, loads ale more evenly distributed across the surfaces of both bushings and there is a reduced chance of the cam shaft contacting a small area or edge of one of the bushings. Such support increases the load-bearing area available for the cam shaft, thus reducing bushing wear due to quasi-static and dynamic load conditions
Such a cam shaft support/enclosure assembly also protects the bushings from environmental contamination One such type of cam shaft support/enclosure assembly includes a cam tube, and the cam tube eliminates two seals over bushings that are used without a cam tube, and reduces by two the number of locations for ingress of contaminants into the bushings. The cam tube also provides a larger grease reservoir to improve lubricity and to suspend any contaminants that may migrate past the seals.
However, prior art cam shaft support/enclosure assemblies typically have required a weld at the attachment point of the inboard end of the cam tube to the vehicle to react rotation of the tube induced by cam shaft rotation Unfortunately, such a weld can be subject to fatigue and failure Moreover, due to the requirement of such welding, and/or potential shimming during installation of the cam tube support/enclosure assembly during production of an axle/suspension system, the cam shaft support/enclosure assembly typically must be custom fit to a single type of axle/suspension system More specifically, different axle/suspension systems have different distances between the two major support points for the cam shaft support/enclosure assembly, namely, the brake system spider and the beam of the axle/suspension system Thus, one size of cam shaft support/enclosure assembly with fixed weld points may fail to fit many axle/suspension systems. In addition, custom-fitting may also be required on same-type axle/suspension systems due to small tolerances in the distance between the support points for the cam tube caused by natural variations in manufacturing processes
In an effort to overcome the limitations associated with welding, it is known in the art to use a cam tube support plate or bracket formed with a predetermined keyhole configuration, such as a D-shape, in conjunction with an O-ring to grip the cam tube and minimize or prevent cam tube rotation. The cam tube support plate keyhole configuration matches the configuration of the periphery of the inboard end of the cam tube The cam tube support plate reacts to cam tube rotation and thus eliminates the possibility of fatigue to and failure of a weld The keyhole slip fit between the cam tube and support plate further enables installation of the cam tube on different types of axle suspension systems, where the distance between the brake spider and inboard suspension assembly point of support for the cam tube varies, as well as on same-type suspension assemblies without concern for differing distances between the support points caused by natural variations in manufacturing processes However, as the O-ring used in connection with such support plates ages, it can become less effective in gripping the cam tube and preventing its rotation In addition, the cam tube support plate keyhole configuration interface with the cam tube may not be enough to secure the cam tube, so relative movement may occur at both the cam-tube-to-suspension-assembly and cam-tube-to-brake-spider interfaces, depending on the condition of certain cam shaft support/enclosure assembly components, such as the O-ring This combined possible failure to satisfactorily limit relative movement of the cam tube may cause undesirable wear on component contact surfaces that, if not corrected, can result in the need to replace the worn components
The present invention solves the above-described problems of possible fatigue failure and custom-fitting by utilizing an improved inboard cam tube support plate or bracket having a generally concave shape prior to assembly, and three or more extruded tabs to create a spring effect and thus exert radial forces on the cam tube, with improved force distribution to secure the cam tube. In addition, the plurality of bracket extruded tabs may be roughened, which in turn adds to the frictional interface between the bracket and cam tube, resulting in increased load capacity and resistance to torsional and axial deflection of the cam tube and reduced fretting Optionally, fewer than three extruded tabs, which include roughened or textured faces, may be utilized without affecting the overall concept or performance of the invention. The present invention provides a more secure attachment of the cam tube to the suspension assembly, and also extends the life of the cam shaft support/enclosure assembly